The invention of high energy product rare earth-iron based permanent magnets has created a need for high yield, low cost processes for making them.
U.S. Pat. Nos. 4,496,395 and 414,936 (filed Sept. 3, 1982) and 544,728 (filed Oct. 26, 1983) all to Croat and assigned to General Motors Corporation relate to this new breed of rare earth-iron (RE-Fe) containing permanent magnets. The preferred magnet compositions contain the light rare earth-elements neodymium and/or praseodymium, the transition metal iron or mixtures of iron and cobalt, and boron in relative amounts such that a substantial amount of a magnetically hardenable RE.sub.2 TM.sub.14 B phase is present.
A preferred method of making such magnets is to rapidly solidify molten alloy such that atomic ordering ranges in the solid are smaller than or about equal to optimum single magnetic domain size (about 400 nanometers). Further processing such as annealing, pressing and/or hot working as taught in the applications noted above and U.S. Pat. No. 520,170 (filed Aug. 4, 1983) to Lee, also assigned to General Motors Corporation, have produced RE-Fe-B magnets with energy products of about forty-five megaGaussOersteds.
One method of rapid solidification is jet casting or melt spinning. This method entails expressing molten alloy through a small orifice (about 0.025-0.05 in., 0.635-1.27 mm) onto the perimeter of a rapidly rotating chill wheel. The molten alloy quenches almost instantaneously to produce very thin, brittle, ribbons having the desired amorphous to very finely crystalline microstructure.
A problem with melt spinning is that the orifice tends to wear and get larger during long runs. Another problem is that a constant source of molten alloy is required to feed the jet casting tundish. It is also necessary to use fairly pure forms of the alloy to prevent plugging of the small ejection orifice with insoluble contaminants.
It was our principal object to find an alternate, high through-put method of rapidly solidifying rare earth-iron based alloys. One such method is plasma spray deposition which is described in the article "Production of Rapidly Solidified Metals and Alloys" by S. J. Savage and F. H. Froes, Journal of Metals, Apr. 1984, pages 20-33 at page 26.
A plasma gun or torch generally comprises a nonconsumable anode and a cathode. An electric arc is struck between the electrodes which ionizes a gas to form an ion plasma. Plasma spray deposition is a method wherein a liquid or powdered metal feedstock is injected into the plasma and is projected towards a substrate at high velocity. The projected metal deposits on the substrate. A layer about 0.1 mm thick may be deposited on each pass of the spray gun. As much as 10 lbs. or 4.5 kg. per hour can be processed through a 40 kWatt plasma torch. The through-put can be increased by using a higher power torch.
We have found that plasma deposition is not directly suited to the manufacture of rapidly solidified RE-Fe magnets other than thin films (less than about 0.2 mm) deposited on a heat conductive metal backing. Repeated passes of the plasma torch have a tendency to over-anneal (i.e., cause substantial crystal growth) in the material built up underneath. This problem is aggravated because RE-Fe compositions are not very good heat conductors and cannot transfer the heat generated by the torch out of the deposited alloy fast enough to create the desired substantially amorphous to finely crystalline microstructure. It was found that "cross blasting" or the process of blowing an inert gas jet transversely into the plasma spray enough pressure to divert the plasma jet without solidifying the molten feedstock droplets or changing their trajectory did not solve the problem of underquenching (over-annealing).